1. Field of the Invention
The present invention relates generally to a glass ceramic substrate containing mica, on which an electrically conductive film is formed, and more particularly to such a glass ceramic substrate having an electrically conductive film secured thereto with a high degree of adhesion, which substrate permits the electrically conductive film to be formed in a desired pattern with improved accuracy.
2. Discussion of the Prior Art
There has been proposed a glass ceramic substrate containing mica, which has an electrically conductive film formed thereon to provide a conductor circuit or electrodes, or to metallize the substrate to permit soldering or brazing. It is recognized that a glass ceramic substrate containing mica is free of open pores, and has excellent properties in terms of electrical insulation, workability, heat insulation, machinability, resistance to corrosion, and so forth. Hence, the glass ceramic substrate having an electrically conductive film exhibits at least one of these properties of the substrate and certain properties of the conductive film, which combine to enable the substrate to attain its intended purpose. For instance, such a glass ceramic substrate may be used as a discharge panel for display units, or a circuit substrate for an angle transducer or other devices, or a thermal recording head as disclosed in laid-open Publication No. 62-238767 of unexamined Japanese patent application. In the publication, the recording head is adapted to apply an electric current to an ink film for effecting electrothermal printing of images on a recording medium.
However, the conventional glass ceramic substrate as described above suffers from a low degree of adhesion of the electrically conductive film with respect to the substrate. To improve the adhesion of the film, the appropriate surface of the glass ceramic substrate containing mica may be roughened to some extent so that the electrically conductive film adheres firmly to the roughened surface of the substrate. While this method is effective to increase the adhesion of the film to the substrate, there still exists another problem that the roughened surface of the substrate may lead to roughening of the surface of the electrically conductive film, thereby affecting the physical properties and configuration of the electrically conductive film. In patterning the electrically conductive film formed on the glass ceramic substrate containing mica by an etching process, it is difficult to provide a sufficient degree of dimensional accuracy for a desired pattern of the film in the etching process, under some adverse influence of mica incorporated in the substrate, even if the surface of the substrate on which the film is formed remains even (i.e., is not roughened). Where the surface of the substrate is made rough for increased adhesion of the film, it becomes further difficult to provide sufficient dimensional accuracy in a pattern forming process of the electrically conductive film, particularly in respect of straightness of the obtained pattern, because of the roughened surface of the substrate in addition to the influence of mica mentioned above.